1. Field of the Invention
The present invention relates to a fast wavelength-tunable laser system, especially to a laser system using a multi-longitudinal-mode Fabry-Perot laser diode as an inter-injection light source and a gain cavity formed by other laser diodes of such type so as to achieve the function of fast wavelength switching.
2. Related Art
Conventionally, a fast wavelength-tunable light source often plays the role of high-speed broadband light source in the applications of wavelength division multiplexing (WDM) system and photonic switching network as well as the application of optical fiber communication.
Currently, there are many efforts made on the research and development of fast wavelength-tunable light source such as U.S. Pat. No. 6,373,867 issued to H. Lin et al. at Apr. 16, 2002 with the title of “Generation of a wavelength-tunable laser oscillation in a wave-guiding gain medium based on passive mode lock” and U.S. Pat. No. 5,284,791 issued to Y. Sakata etal. at Feb. 8, 1994 with the title of “Method of making tunable semiconductor laser”. However, in above documents, it is necessary to design a complicate structure in accordance with the application due to a limited tuning range resulted from the output of various multi-mode laser diodes.
Generally, a wavelength-division multiplexing (WDM) system refers to an optical communication system capable of transmitting several separate modulated light signals in one single optical fiber due to signals from light source with different center frequency are not overlapped but shifted by appropriate amounts respectively and can be separated by using optical filters. The light source usually used in such system is the Fabry-Perot laser diode making use the principle of Fabry-Perot interference, in which a standing wave is formed from electromagnetic interference between waves reflected by two mirrors of a resonance cavity due to the enhancement occurred on some frequencies while the attenuation occurred on the other frequencies. However, the disadvantage of such laser is the tuning range is constrained to the output of various multi-mode laser diodes so that Fabry-Perot laser diodes with different center wavelengths must be provided to obtain output of different frequency.
Therefore, it is necessary to develop a laser system required only a simple wavelength tuning technique instead of a process related technique to select different Fabry-Perot laser diodes in accordance with different wavelength requirement so as to achieve fast wavelength switching while having the side-mode suppressing ratio and modulation range of a wavelength-division multiplexing (WDM) system is fully compatible to the available gain bandwidth of a fiber amplifier.